1. Field of the Invention
The present invention relates to electrical circuits, and more particularly, to techniques for monitoring the performance of circuits and replacing low performance circuits with higher performance circuits.
2. Description of Related Art
Integrated circuits (ICs) typically include numerous transistors such as field effect transistors (FETs). A FET turns on when an input voltage reaches its threshold voltage.
During the operation of an integrated circuit (IC) chip, the threshold voltages of transistors that are switching shift over time. The shift in threshold voltage is caused by the operating temperatures on the IC. Positive bias temperature instability (PBTI) causes the threshold voltages of n-channel MOSFETs to shift. Negative bias temperature instability (NBTI) causes the threshold voltages of p-channel MOSFETs to shift.
NBTI and PBTI can cause a relatively significant change in the threshold voltages of nanoscale CMOS technology. The shift in the transistor's threshold voltage can become significant enough to cause serious performance and reliability problems in advanced chip applications.
NBTI worsens at high operating temperatures, causing p-channel MOS threshold voltages to shift on the order of a few tenths of a mV. A change of a few millivolts can significantly degrade circuit performance and lead to functional failure. For example, if the threshold voltage of n-channel transistors increase due to PBTI, less current will flow through the transistors when their gate voltages are raised to a predefined point. As a result, the transistors will switch more slowly, and circuit performance degrades.
Stable transistor threshold voltages are a primary requirement for certain high precision analog circuitry. If the circuitry that generates an on-chip clock signal slows down, the clock signal frequency may decrease, slowing down large segments of circuitry on the chip. A reduction in the clock signal frequency can cause an unacceptable reduction in the chip's operating speed.
Therefore, it would be desirable to reduce the performance and reliability problems caused by the shift in transistor threshold voltages due to NBTI and PBTI.